In general, in a conventional method of operating a particle microscope, a sample is positioned at a given distance from a magnetic objective lens of the microscope and a voltage between a particle source and the sample is set to a given value. This voltage is involved in determining the kinetic energy of the particles impinging on the object. Thereafter, generally, a lens current supplied to the objective lens has to be adjusted such that the particle beam is focused at the sample surface. Typically, the value of this lens current achieving the focus is initially not known because the precise distance of the sample from the objective lens is typically not known. A procedure of determining a lens current optimized for achieving the focus of the particle beam at the sample is usually performed.
For example, the lens current can be adjusted to different values, and, at each of the different lens current values, image information of the sample is obtained using the particle beam system. The image information is analyzed to determine a contrast, and next lens current values are determined based on the determined contrast and previous settings of the lens current in an iterative search process until a satisfactory optimized lens current is found.
It is to be noted that, generally, the magnetic objective lens has a high inductance such that it is not possible to rapidly change the lens current to a desired value. Therefore, the above process is, in general, time consuming.
According to another procedure, the lens current is continuously changed from a low value to a high value or from a high value to a low value, wherein the high and low values are selected such that the expected optimized lens current is in between these values. Image information is continuously recorded during such scan of the lens current between the low and high values. If the sample is sufficiently structured, the image information will show a peak contrast at a time during the scan where the beam is focused at the object. The lens current supplied to the objective lens at that time could be used as the optimized lens current. However, if the lens current is set to this optimized lens current and images of the sample are recorded at this optimized lens current, generally, it appears that a nearly optimal focus adjustment has not been achieved and that the focus can be further improved.